The Insulated-Gate Bipolar Transistor (IGBT) is a three-terminal power semiconductor device primarily used as an electronic switch and in newer devices is noted for combining high efficiency and fast switching. It switches electric power in many modern appliances such as: Variable-Frequency Drives (VFDs), electric cars, trains, variable speed refrigerators, lamp ballasts, air-conditioners, and even stereo systems with switching amplifiers.
IGBTs are often used for high voltage (e.g., greater than 600V) and high-current power converter applications. However, turning off IGBT in the system 100 of FIG. 1 becomes a tricky task because of the influence of IGBT's parasitic inductances on system behavior. The current change by turning off the IGBT 108 will cause voltage spikes at its collector.
During normal powering down of the load 112, in order to achieve overall system efficiency, one has to keep the IGBT 108 switching energy loss minimum and Miller plateau short. In many implementations, a small gate resistor Rg and/or external buffer are used to achieve fast (hard) shut-down.
For higher overcurrent or short circuit conditions, a large gate resistor Rg is desired for slow (soft) shut-down to prevent the collector-to-emitter voltage of the IGBT 108 (Vce) transient from getting too high to exceed its absolute maximum rating. Unfortunately, this will result in an extended turn-off Miller plateau and higher switching losses for normal switching conditions. Hence, overall system efficiency is reduced.
As shown in FIG. 2, some solutions employ a small resistor Rg to turn off for all current conditions, with the help of active clamping circuit 204 to prevent IGBT 108 from breakdown. Active clamping is implemented by feeding back the collector potential to the gate via an element 204 with avalanche characteristic, such as TVS.
A problem with such an approach is that the addition of the clamping device 204 reduces the effective working voltage of IGBT 108 and hence reduces operating efficiency. Another problem is that when the clamping device 204 is activated, high power loss is introduced by the device itself. High power dissipation of clamping device 204 will affect its lift time, and hence causes reliability issues to the driver system 200.